Printable inks that fluoresce under ultraviolet A (UV-A, or black light) illumination, that is, ultraviolet light having wavelengths of 315-400 nm, are increasingly available in the market. Such inks are referred to herein as one example of “UV ink.” Some UV inks are also visible when illuminated by white light, that is, light having wavelengths of 400-700 nm, and are referred to herein as “visible UV inks.” Other UV inks have little or no color under white light illumination and are referred to herein as “invisible UV inks.” Both visible UV inks and invisible UV inks can be used in combination with conventional white light visible inks, referred to herein as “visible inks” that do not fluoresce, thereby allowing the creation of images that can be seen in both standard white light and under UV-A illumination.
The huge variety of conventional visible inks and growing variety of UV inks and well understood mixing techniques give artists the ability to create spectacular effects such as invisible images, dual images, and day/night transitioning images. However, UV inks also present new artistic challenges.
Properties that affect the appearance of UV inks to the human eye include, like visible inks, hue (color) and saturation (value). The UV inks have the additional variability of luminosity. Luminosity with respect to UV inks generally refers to the amount of light emitted by the fluorescent particles in the UV ink when it is illuminated with UV-A light. For the most part UV inks are created to maximize luminosity as it is generally thought preferable to have the inks emit as much light as can be produced. Further, the color palette of UV inks is limited to what appears to be neon type colors due to the emissive nature of the inks.
The distinct lack of mid-range tones and color palette range available in known UV ink printing processes is caused by the lack of ability to control the luminosity of the color. As ever higher luminosity inks are developed, the images that can be produced with the inks remain limited by this lack of control. Controlling the luminosity allows the artist to create the illusion of depth. Currently, visible black ink is used to decrease luminosity of UV inks by covering up some portion of the UV ink imaged area, since black visible ink does not fluoresce and thus appears black under both visible and UV-A illumination. However, this approach suffers from the problem that the black ink also appears black in white light conditions, and thus darkens the appearance of the visible ink image. Unlike dye-based inks, black ink is pigmented, thus is opaque. The effect of an opacifying agent is to affect luminosity in a manner that does not enable mid-range tones or a full color palette.
Since visible black ink affects both UV luminosity and white light shading at the same time, the artist is forced to compromise the resultant image. It would be desirable to control luminosity of the UV inks and shading of the visible inks independently to enable the production of dual images with improved appearance.
An additional level of complication arises when printing with visible UV inks. In such cases, the appearance of the image as illuminated by white light is determined by the combination of visible UV ink and visible ink. Thus, once the visible color is selected, the UV-A luminosity of the image is fixed. If the artist desires to modify the UV-A luminosity, she may compromise the appearance of the image as illuminated by white light.
An illustration of this problem is provided by an artist who wants to make an image having light blue shading that fades to white, and wants that image be visible both in white light and UV-A light. Currently, to fade the UV blue ink the artist would add visible black (also visible as black under UV-A light) to reduce the luminosity. This only causes the image to fade to black. Further, the presence of visible black pigment ruins the fade-to-white present in the visible light image. Finally, since the presence of black pigment is visible in white light, UV ink images intended to be invisible in white light are not enabled on anything but a black background.
This problem is also manifested in the inability of the artist to provide the subtle mid-range tones required to give the illusion of depth in a two-dimensional printed image. Addition of mid-range tones to UV ink images would provide the artist with the ability to create the nuances and enable effective illusion of depth in printed images.
Accordingly, there is a need in the industry for printing systems, printing methods, and ink compositions that enable luminosity control in UV inks with minimal impact on the appearance of white light printed matter.